A battery-based power supply typically includes a protection circuit for detecting battery conditions, so as to prevent dangerous operations such as over charging, over discharging, and over-current operation while the battery is being charged or discharged. Conventionally, as shown in FIG. 1, a battery package 10 for portable electronic devices includes a battery 12, MOSFETs M1 and M2, and a protection circuit 14. The MOSFETs M1 and M2 have body diodes D1 and D2, respectively, and the body diodes D1 and D2 are arranged back to back. The protection circuit 14 detects the voltage of the battery 12 and turns on or off MOSFETs M1 and M2 accordingly, thereby controlling a current flowing into or out of the battery package 10. A resistor 16 and a current source 18 represent the load and a charger, respectively. During charging operation, the current source 18 provides a charge current for charging the battery 12; during discharging operation, the battery 12 provides a discharge current to the resistor 16.
During normal charging or discharging operation, both the MOSFETs M1 and M2 are turned on such that the charge current and the discharge current flow through the same path, namely by way of the MOSFETs M1 and M2; besides, since the voltage drop across the MOSFET M1 as well as the MOSFET M2 is small, neither of the body diodes D1 and D2 is conducted. However, when the protection circuit 14 detects that the battery 12 is over charging, the MOSFET M1 will be turned off while the MOSFET M2 will be turned on. Thus, the body diode D1 blocks the charge current and only allows the discharge current to pass through. On the other hand, when the protection circuit 14 detects that the battery 12 is over discharging, the MOSFET M1 will be turned on, and the MOSFET M2 will be turned off. Consequently, the body diode D2 blocks the discharge current and only allows the charge current to flow through.
However, the conventional method described above needs two MOSFETs, i.e., M1 and M2, which result in high costs and require a large die area. Moreover, when the discharge current or the charge current flows through the body diode D1 or D2, overheating and power loss occur.
Therefore, it is desired a battery protection apparatus capable of reducing costs, decreasing die area, avoiding overheating, and reducing power loss.